The production of an extra low carbon steel by a melt process has now become easy by virtue of advances in vacuum degassing processes for molten steels in recent years. This has led to an ever-increasing demand for extra low carbon steel sheets having a good fabricability.
It is well known that the extra low carbon steel sheet of this type generally contains at least one of Ti and Nb. Specifically, Ti and Nb interact with interstitial solid solution elements (C, N) in the steel, by a strong force of attraction, to easily form carbonitrides. Therefore, the resultant steel is a steel free from interstitial solid solution elements (IF steel: interstitial free steel). Since the IF steel does not contain an interstitial solid solution element causative of a deterioration in strain aging and formability, it has a feature that the formability in a non-aging state is very good. Further, addition of Ti and Nb plays an important role in improving deep drawability of the cold-rolled annealed steel sheet by virtue of grain refining of a hot-rolled sheet of an extra low carbon steel which is likely to coarsen. However, the extra low carbon steel containing Ti and Nb has the following problems. First, the production cost is high. Specifically, in addition to a vacuum treatment cost for rendering the carbon content extra low, addition of expensive Ti and Nb is necessary. Second, since neither C nor N in a solid solution form remains in the product sheet, fabrication embrittlement occurs or the paint-bake hardening disappears. Thirdly, Ti and Nb are powerful oxide forming elements, and the formed oxides deteriorate the surface quality.
Much research and development has hitherto been made with a view to solving the above problems of IF steel. For example, Japanese Unexamined Patent Publication (Kokai) Nos. 60-197846 and 63-72830 disclose extra low carbon steel sheets containing neither Ti nor Nb and process for producing the same, and fundamentally, the above problems are solved by the following technique. In continuously annealing a steel having a C content in the range of from 0.0010 to 0.0080%, high-temperature annealing is used to transform part of .alpha. to .gamma., and a low-temperature transformation product from .gamma. is then formed by regulating the cooling rate to provide a mixed structure comprising .alpha. and .gamma.. Since, however, the (.alpha.+.gamma.) two-phase region of the extra low carbon steel is very narrow, it is difficult to regulate the temperature with a high accuracy. In addition, there occur various problems associated with annealing at a high temperature, for example, a poor travelability of the sheet at a high temperature, a poor sheet configuration and a high energy consumption. For this reason, the steel sheet of the present invention comprises a structure with a single .alpha. phase. Further, Japanese Unexamined Patent Publication (Kokai) Nos. 59-80727, 60-103129 and 1-184251 and the like disclose cold-rolled steel sheets containing none of expensive elements such as Ti and Nb and having a C content including a region of not more than 0.0015% and a process for producing the same. In these steel sheets, however, the addition of B, which is one of the features of the present invention, is not conducted. When the total C content is not more than 0.0015%, the amount of C present in grain boundaries is extremely small even though neither Ti nor Nb is added, which gives rise to fabrication embrittlement. Further, in Japanese Unexamined Patent application (Kokai) No. 58-141335, the C content range includes a region of 0.0015% or less, and B is added in an amount in the range of from 0.0005 to 0.0020%. However, When the C content is in a region of 0.0015% or less, the crystal grain diameter of the hot-rolled steel sheet is generally so large that the r value (average Lankford value) of the cold-rolled annealed sheet cannot be ensured. For this reason, it becomes necessary to take some measures for elements added and a hot-rolling method.
On the other hand, in the production of a hot-dip galvanized steel sheet using continuous hot-dip galvanizing equipment by the sendzimir method, if the steel contains elements, which easily form oxides, for stabilization, such as Ti and Nb, an oxide film is likely to remain on the surface of the steel sheet even when the steel sheet is subjected to reduction before plating. This oxide film has an effect both on the wettability of the steel sheet by plating and on the alloying reaction of Fe with Zn, which makes it difficult to stably produce steel sheets having a high quality.